1. Field of the Invention
The invention relates to polyalcohol mixtures obtained by the dehydration of D-sorbitol, L-sorbitol or mixtures thereof. Specifically, the invention relates to a method of obtaining a specific polyalcohol mixture by monitoring the optical rotation of the dehydration reaction.
2. Discussion of the Background
Emulsifiers based on sorbitan esters, which demonstrate very specific hydrophilic and hydrophobic properties and can be completely synthesized on the basis of renewable raw materials are increasingly used in practice. It is known that the property profile of these esters is decisively influenced by the composition of the polyalcohol mixture on which they are based, for example, sorbitol, monoanhydrosorbitol (sorbitan), dianhydrosorbitol (isosorbide), etc. For this reason, there is interest in processes which make it possible to produce specifically adjusted polyalcohol mixtures based on sorbitol.
Various processes are known for producing anhydro-polyls starling from D-sorbitol (see e.g. B. R. Barker, J. Org. Chem., 35, 461 (1970), J. Feldmann et al., EP-OS 0 052 295 and DE-OS 30 14 626, Soltzberg et al., J. Am. Chem. Soc., 68, 919, 927, 930 (1946) and S. Ropuszinski et al., Prozed. Chem, 48, 665 (1969)). In all these processes, water is split off intramolecularly, in the presence of an acid catalyst and at a raised temperature, and mixtures of anhydrous forms are always obtained. The polyalcohol mixture obtained by dehydration consists, for example, according to K. Bock et al. (Acta Chem. Scand., 835, 441-449 (1981)), of 41.9% D-sorbitol, 49.0% 1,4-anhydro-D-sorbitol, 2.4% 3,6-anhydro-D-sorbitol, 3.7% 2,5-anhydro-L-iditol, 1.0% 2,5-anhydro-D-mannitol, and 2.1% 1,4:2,6-dianhydro-D-sorbitol.
As the reaction progresses, the concentration of dianhydrosorbitol increases, while that of sorbitol decreases.
Until now, all efforts to stop the dehydration at the stage of monoanhydrosorbitol, or to continue the reaction until exclusively dianhydrosorbitol is formed, have failed.
Defined polyalcohol mixtures can basically be produced by mixing the components. This procedure, however, presupposes cleaning and isolation of the components. But since only mixtures occur in all cases and separation is difficult, this method of procedure is extremely complicated and requires great effort.
A possibility of producing such mixtures in a targeted manner is proposed in DE-0S 30 41 626. According to this reference, the dehydration reaction time is selected in such a way that a certain hydroxyl number is reached. The process of G. J. Stockburger in U.S. Patent 4,297,290 provides for heating sorbitol in the presence of toluenesulfonic acid until an OH number of 1,195 has been attained in the reaction mixture, and subsequently carrying out esterification in the presence of sodium hydroxide. Determination of the 0H number is known to be an experimental procedure which requires a lot of time. It is not suitable for directly following the progress of a reaction. Rather, it is only possible, in preliminary experiments, to determine the point in time at which a certain 0H number will presumably be reached. A targeted quenching of the reaction is not possible in this way. A number of factors, such as, for example, temperature, pressure, stirring velocity, concentration ratios, nitrogen bubbling velocity, relative surface, batch size, water content and quality of the initial substances, significantly influence the reaction velocity. The process is therefore only reproducible under strictly identical conditions. These procedures are therefore not suitable for practical situations, where such parameters undergo contiual slight changes. Therefore, it is disclosed in DE-0S 30 41 626that for each individual case, the optimum reaction time first has to be determined (c.f. column 6, line 59-65).
There have been efforts to determine the reaction progress directly by following the reaction parameters over time. E. Soltys et al., Tulszcze, Srodki Piorace, Kosmet, 13, 48 (1969), Chem. Abstr. 71, 124 831 (1969) describe such a method. The viscosity is used as a measure of the reaction progress. It is known, however, that the viscosity is significantly dependent on the concentration of the solvent, in this case water. But since the concentration of the water is specifically dependent on the factors which also influence the reaction velocity, this method is not suitable for following the progress of the reaction.
Fleche and Huchette follow the kinetics of the dehydration reaction using high-pressure liquid chromatography. This process can only be carried out discontinuously. Although it does yield fairly reliable values, it is technically complicated and requires a significant amount of time. It is therefore also undesirable for following the reaction on a technical scale (see Starch/Starke 38, 26-30 (1986)).